1. Field of the Invention
The present invention concerns a procedure for controlling a radiation source, said radiation source being implemented with the aid of light-emitting diodes, or LEDs, of the radiation produced by them the desired wavelength range being separated and the intensity of this radiation range being controlled or kept constant.
The present invention also concerns a controllable radiation source which has been composed of light-emitting diodes, or LEDs, and which comprises optical means for separating the desired wavelength range from the radiation produced by said LEDs and means for controlling the intensity of said wavelength range or keeping it constant.
2. Description of the Prior Art
A radiation source of the kind described is used for instance in spectrometers and photometers. In frequent cases the radiation source is the most significant factor limiting the capacity of performance and usability of an instrument. In apparatus meant to be used in industrial conditions, as radiation source have usually been employed thermal radiators, such as sources based on an incandescent filament, for instance. Their problem is, however, poor optical efficiency and consequent high heat dissipation, as well as poor vibration tolerance, short service life and difficulty of modulation.
In recent years the development of semiconductor technology has introduced on the market efficient lasers based on semiconductor junctions, and light-emitting diodes, or LEDs. They afford several advantages over traditional radiation sources: for instance, small size and low energy consumption, good reliability, long service life (even more than 10.degree. hours), high operating speed, easy connection to optic fibres. Furthermore, they can be electrically modulated with ease. Semiconductor radiation sources are nowadays available for the wavelength range about 400 nm to over 10 .mu.m; admittedly, though, for operation at room temperature only up to about 3200 nm. The said range is usable in quantitative and qualitative analysis of most substances.
Semiconductor lasers are nearly ideal radiation sources for spectrometers in view of their narrow spectrum. However, high price and poor stability are their problems. It is also a fact that the selection of standard wavelengths is scanty, particularly in the near IR range. LEDs enable, owing to their wider radiation spectrum, a considerably wider wavelength range to be covered, and they are also lower in price. The spectral radiance of LEDs is on the same order as that of most thermic radiation sources, or higher.
The radiations spectrum of LEDs is mostly too wide to allow them to be used as such in spectroscopic measurements. Moreover, the shape of the radiation spectrum, the peak wavelength and the radiant power change powerfully with changing temperature and driving current, and with time.
In spectrometer designs of prior art based on the use of LED sources, the measuring band is separated from the LEDs, usually, with the aid of separate filters, or the LED is used without filtering, in which case the resolution will also be poor. The variation of radiation intensity has most often been compensated for, either by mere electric compensation or by maintaining constant temperature of the LED, which has lead to a demanding and expensive mechanical design. Owing to the high price and difficult manipulation of the filters (e.g. miniaturizing, cutting), the number of wavelength bands in such pieces of apparatus is usually small (2 to 10).